The present invention relates to brake shoes for railroad cars and particularly to composition lined brake shoes having a metal backing plate to which the composition brake shoe material is bonded.
Brake shoes of the aforementioned type typically incorporate a projecting structure of the backing plate known as a key bridge for insertion into a complementary pocket in a brake head or brake shoe carrier via which a locking key is passed to firmly secure the brake shoe to the brake head under spring tension of the locking key. Depending upon the particular style of manufacture, the brake lining of these brake shoes may be continuous over the entire braking face or alternatively, may be discontinuous. In the latter case, the discontinuity in the braking face may be manifested by a recess formed in the braking face substantially adjacent the keybridge concavity that only extends partially into the composition brake lining, or by a two-piece lining that results from the recess extending completely through the brake lining into the keybridge concavity.
Collapse and breakage of the backing plate in the critical area of the keybridge is possible in these various style brake shoes. Vibration and impacts that occur in service between the brake shoe and brake head due to irregular track and/or damaged wheel treads cause excessive brake head wear at the interface with a brake shoe. As this brake head wear increases, flexure of the brake shoe backing plate can occur during application of the brakes. In time, collapse and breakage of the backing plate occurs in the area of the key holes in the keybridge as a result of this flexure. Breakage of the backing plate in this area is considered critical since a portion of the brake shoe is free to fall off of the brake head. This not only reduces the brake effectiveness, but if the remaining part of the brake shoe is not replaced soon after the breakage, contact between the wheel and brake head during brake applications could damage these parts.
In the case of a brake shoe having a two-piece lining, it is quite evident that without any brake lining support at the mouth of the keybridge, flexure can occur at the keybridge with consequent breakage, as discussed.
Even brake shoes having a one-piece brake lining that extends intact across the mouth of the keybridge are susceptible to backing plate failure at the keyhole area of the keybridge under conditions of severe vibration and impact forces between the brake head and brake shoe. Moreover, brake shoes having the keybridge concavity filled with composition brake lining material to reinforce the keybridge, as disclosed in the referencing application, are not entirely free of failure, since any cracks that develop in the brake lining adjacent the keybridge can easily migrate to the area of maximum flexure, i.e., the keyhole area of the keybridge. This in effect degrades the integrity of the keybridge reinforcement brought about by filling the keybridge with composition brake material, and allows keybridge flexure and consequent fatigue fracture to occur at the keyhole area.